Method for generating duobinary signal and optical transmitter using the same method

ABSTRACT

Disclosed is a method for generating a duobinary signal which has the step of: modulating individually an intensity and a phase of a carrier wave. Also disclosed is a duobinary-manner optical transmitter which has: a laser device which outputs signal light; an optical intensity modulator which intensity-modulates the signal light according to a first data signal generated by dividing a data signal into two signals; a precoder which inputs a second data signal generated by dividing the data signal into two signals; and an optical phase modulator which phase-modulates the intensity-modulated signal light according to a signal which is obtained delaying 0.5 bit an output signal of the precoder.

FIELD OF THE INVENTION

This invention relates to a method for generating a duobinary signal andan optical transmitter using the same method.

BACKGROUND OF THE INVENTION

Recently, an optical duobinary technique has attracted attention as anoptical transmission manner which can overcome the waveformdeterioration due to a chromatic dispersion. The duobinary techniqueitself has been researched for a long time and its theory system wasestablished in the time of pulse communication with a coaxial cable. Theduobinary technique is that a signal bandwidth (spectrum width) isreduced to less than 1/2 by mapping a binary data signal to betransmitted into a three-level signal with a redundancy in the amplitudedirection. It has a merit that the waveform deterioration due to adispersion etc. is difficult to happen since the spectrum width isnarrowed. However, it had never attracted attention in high-speedoptical communication since, in the receiver, a receiving circuit with alinearity is required to handle the three-level signal and a decoder fordecoding the original binary data signal from the three-level signal isnecessary.

A. J. Price et al., "210 km Repeaterless 10 Gb/s Transmission ExperimentThrough Nondispersion-Shifted Fiber Using Partial Response Scheme", IEEEPHOTONICS TECHNOLOGY LETTERS, Vol. 7, No. 10, pp. 1219-1221(1995)reports an optical duobinary technique where a redundancy is given tooptical phase.

The optical transmitter used in this optical duobinary technique isshown in FIG. 1. A binary data signal is passed through a low-passfilter, which is ideally a cosine roll-off filter, with a bandwidth ofabout 0.25 times a clock frequency. Due to the excessive limitation ofbandwidth, the interference between codes is occurred to convert thebinary data signal into a three-level data signal. Similarly, a binaryinverted data signal is converted into a three-level data signal. Then,these signals are input with an amplitude equal to a half-wavelengthvoltage V.sub.π to a push-pull optical intensity modulator. Thepush-pull optical intensity modulator is a Mach-Zehnder(MZ)interferometer with modulation terminals connected to both arms, whereunnecessary chirp(phase variation) does not occur. In this technique,the bias voltage is so adjusted that a three-level signal(-1, 0, 1)corresponds to a mountain(ON), a valley(OFF) and a neighboringmountain(ON) in the voltage-extinction characteristic of the push-pulloptical intensity modulator. As a result, when the amplitude and phaseof light are represented by (A, Φ), the data signal is mapped into threestates of (1, 0), (0, indefinite) and (1, π) to generate opticalduobinary signal light. This three-level signal light can be, as it is,decoded into the binary signal composed of 1 and 0 since the phaseinformation is deleted by square-law detection when the direct detectionis conducted by an optical detector. This means that direct-detectionoptical receivers, which are widely used, can be used as it is. It isone of the reasons why the duobinary technique has attracted attentionagain.

Japanese patent application laid-open No.8-139681(1996) disclosesanother optical duobinary system as shown in FIG. 2. In this system, asshown in FIG. 2, a binary transmission data signal 50 is converted intoa three-level duobinary signal by a code converter 51. In the codeconverter 51, the code conversion is first conducted by a precoder 52composed of an exclusive-OR circuit 26 and an 1-bit delay circuit 27,and then the duobinary signal is generated by a binary-to-three-levelconverter 53 composed of an 1-bit delay circuit 27 and an adder 54. Theduobinary signal is divided into two signals, where the first signaldivided is input through an amplitude adjusting circuit 55 and a biasadjusting circuit 56 to the first input terminal of an optical modulator58 and the second signal divided is input through an inverter 57 and anamplitude adjusting circuit 55 to the second input terminal of theoptical modulator 58. The optical modulator 58 is a Mach-Zehnder opticalintensity modulator, where light from a light source 1 is modulated byapplying the first and second signals to its two optical A waveguides togenerate the optical duobinary signal.

When the two electrical signals with an amplitude equal to ahalf-wavelength voltage(V.sub.π) of the optical modulator 58 are inputand the bias point of signal is set at a point (a) of transmissioncharacteristics 59 of the modulator as shown in FIG. 3, the middle valueof the duobinary signal 60 is assigned to a minimum transmittance stateand the minimum and maximum values thereof are assigned to maximumtransmittance states, where the optical phase is inverted by 180 degreebetween the minimum and maximum values. As a result, the three levels ofthe electrical signal can be assigned to the optical three states,thereby narrowing the modulated light spectrum. Meanwhile, this systemhas a composition equivalent to the system in FIG. 1 where the low-passfilters are replaced by the binary-to-three-level converter 53.

However, in the conventional methods, the driving amplifier of themodulator requires a linearity since the electrical signal for drivingthe optical modulator is three-level. On the other hand, the drivingamplifier generally needs a high-output characteristic greater than 5Vp-p. Therefore, there is a problem that designing the circuit becomesvery difficult since the linearity and the high-output characteristicare required therein.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide a method forgenerating a duobinary signal where an electrical signal for driving anoptical modulator is binary.

It is a further object of the invention to provide an opticaltransmitter where a binary electrical signal for driving an opticalmodulator is used to generate a duobinary signal.

According to the invention, a method for generating a duobinary signal,comprises the step of:

modulating individually an intensity and a phase of a carrier wave.

According to another aspect of the invention, a duobinary-manner opticaltransmitter, comprises:

a laser device which outputs signal light;

an optical intensity modulator which intensity-modulates the signallight according to a first data signal generated by dividing a datasignal into two signals;

a precoder which inputs a second data signal generated by dividing thedata signal into two signals; and

an optical phase modulator which phase-modulates the intensity-modulatedsignal light according to a signal which is obtained delaying 0.5 bit anoutput signal of the precoder.

According to another aspect of the invention, a duobinary-manner opticaltransmitter, comprises:

a precoder which inputs a second data signal generated by dividing adata signal into two signals;

a direct modulation phase shift keying encoder which inputs an output ofthe precoder after delaying 0.5 bit the output;

a laser device which outputs signal light phase-modulated by modulatingan injected current according to an output of the direct modulationphase shift keying encoder; and

an optical intensity modulator which intensity-modulates thephase-modulated signal light according to a first data signal generatedby dividing the data signal into two signals.

According to another aspect of the invention, a method for generating aduobinary signal, comprises the steps of:

providing two carrier waves with an equal frequency;

intensity-modulating individually the two carrier waves by first andsecond intensity modulators; and

coupling the two intensity-modulated carrier waves so that they have aphase difference of π.

According to another aspect of the invention, a duobinary-manner opticaltransmitter, comprising:

a laser device which outputs signal light;

an optical divider which divides the signal light into two lightsignals;

a first optical intensity modulator which inputs first signal lightdivided by the optical divider;

a second optical intensity modulator which inputs second signal lightdivided by the optical divider;

an optical coupler which couples output lights of the first and secondoptical intensity modulators after phase-shifting at least one of theoutput lights so as to give a phase difference of π between the outputlights of the first and second optical intensity modulators; and

a precoder which inputs a data signal;

wherein the first optical intensity modulator is driven by a firstencoded signal generated by dividing an encoded signal to be output fromthe precoder into two signals, and the second optical intensitymodulator is driven by a signal which is obtained by delaying 1 bit asecond encoded signal generated by dividing the encoded signal into thetwo signals, thereafter inverting 0 and 1 each other.

According to another aspect of the invention, a duobinary-manner opticaltransmitter, comprises:

a laser device which outputs signal light;

an optical intensity modulator which intensity-modulates the signallight according to a first data signal generated by dividing a datasignal into two signals;

a precoder which inputs a second data signal generated by dividing thedata signal into two signals; and

an optical phase modulator which phase-modulates the intensity-modulatedsignal light according to a signal which is obtained delaying 0.5 bit anoutput signal of the precoder;

wherein a waveform of the signal light is varied by changing anoperating point of the optical intensity modulator.

According to another aspect of the invention, a duobinary-manner opticaltransmitter, comprises:

a laser device which outputs signal light;

an optical intensity modulator which intensity-modulates the signallight according to a first data signal generated by dividing a datasignal into two signals;

a precoder which inputs a second data signal generated by dividing thedata signal into two signals; and

an optical phase modulator which phase-modulates the intensity-modulatedsignal light according to a signal which is obtained delaying 0.5 bit anoutput signal of the precoder;

wherein a waveform of the first data signal is varied through anon-linear electric circuit.

According to another aspect of the invention, a method for generating aduobinary signal, comprises the step of:

modulating individually an intensity and a polarization of a carrierwave.

According to another aspect of the invention, a duobinary-manner opticaltransmitter, comprises:

a laser device which outputs signal light;

an optical intensity modulator which intensity-modulates the signallight according to a first data signal generated by dividing a datasignal into two signals;

a precoder which inputs a second data signal generated by dividing thedata signal into two signals; and

an optical polarization modulator which polarization-modulates theintensity-modulated signal light according to a signal which is obtaineddelaying 0.5 bit an output signal of the precoder.

In the invention, an intensity modulator and a phase modulator arecascade-connected, and the amplitude(or intensity) and phase of signallight are individually modulated. Meanwhile, an intensity modulationsignal and a phase modulation signal are input to the intensitymodulator and phase modulator, respectively, while having predeterminedconversion and phase relations. The conversion and phase relations willbe explained below. FIG. 4 shows calculation results of the amplitudeand phase of optical duobinary signal light modulated by a conventionalthree-level signal. As shown in FIG. 4, in the optical duobinary signallight, the phase is inverted from 0 to π or from π to 0 at a point wherethe amplitude is 0. The phase inversion occurs at the middle point of a1 time slot. The phase does not change when the amplitude is 1. Thecharacteristic that "the phase is inverted at a point where theamplitude is 0" gives the characteristics of optical duobinary mannerthat have a narrowed optical spectrum and a high durability againstdispersion.

When a data signal is transmitted carrying on an optical amplitude(orintensity) and is directly detected by an optical receiver to get thedata signal, only the phase modulation signal needs to be encoded by aprecoder(encoder). The rule of the encoding is, as described earlier,that "the phase modulation signal is inverted when the intensitymodulation signal is 0". This can be easily achieved by EX-NOR(invertedoutput of exclusive "or") and a 1-bit delay circuit, as shown in FIG. 9.The precoder uses an output value 1 bit before, therefore the output isinverted depending on an initial output value. However, there is noproblem because an absolute optical phase has no meaning for opticalduobinary signal light. After delaying 0.5 bit the phase modulationsignal to the intensity modulation signal, the intensity-modulatedsignal light is phase-modulated(0-π). Thereby, the optical duobinarysignal light can be generated. In such cascade modulation, any one ofthe optical phase modulator and the optical intensity modulator may beplaced at the first.

Also, two lights with an equal frequency may be provided by, forexample, dividing signal light into two lights, then turning OFF(or ON)either of the two lights or both of them by two optical intensitymodulators, thereafter coupling them to give a phase difference of πbetween the outputs from the two optical intensity modulators. As aresult, the data signal can be mapped into three optical states asdescribed earlier to generate optical duobinary signal light(paralleltype). In this case, a parallel-type precoder is necessary for the inputdata signal in addition to the above-mentioned precoder. Theparallel-type precoder can be, as shown in FIGS. 20 and 21, composed ofa simple circuit using a 1-bit delay circuit and an inverter.

Also, in the invention, by properly setting the optical intensitymodulation waveform, modulated light closer to that in the opticalduobinary modulation manner using the three-level signal can beobtained. In an ideal optical duobinary waveform, the cross point isbiased upward as shown in FIG. 5A, where the optical spectrum has verysmall high-frequency components as shown in FIG. 5B. To suppress thehigh-frequency components, the invention approaches the ideal opticalduobinary by using a non-linear modulation characteristic of the opticalintensity modulator or an electric circuit with a non-linear inputcharacteristic. When a signal 62 is input as shown in FIG. 6A, theobtained modulation light spectrum has high-frequency components to beremained as shown in FIG. 7A. On the contrary, when a signal 64 is inputshifting the bias value, the output optical waveform 65, which is biasedto the side of optical transmission, becomes closer to the idealduobinary waveform, and the high-frequency components can also besuppressed as shown in FIG. 7B.

On the other hand, in the composition of cascade-connected intensitymodulator and phase modulator, when the main axis direction ofpolarization of output signal light is supplied to be 45° to the opticalaxis of the phase modulator, only the signal light component in theoptical axis direction is phase-modulated. Therefore the polarization ofsignal light can be modulated according to phase modulation. As aresult, the spectrum of the signal light is not so narrowed, but thepolarization-modulated duobinary signal can be generated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in more detail in conjunction with theappended drawings, wherein:

FIG. 1 is an block diagram showing a conventionaloptical-duobinary-manner optical transmitter,

FIG. 2 is an block diagram showing another conventionaloptical-duobinary-manner optical transmitter,

FIG. 3 is a diagram showing the setting of an operating point of LNmodulator,

FIG. 4 is a diagram showing the calculation results of amplitude andphase of optical duobinary signal light generated by a conventionalmethod,

FIGS. 5A and 5B are diagrams showing an ideal duobirary signal waveformand its spectrum,

FIGS. 6A and 6B are diagrams showing the setting of two kinds of biaspoints in optical intensity modulation,

FIGS. 7A and 7B are diagrams showing the optical spectra to the twokinds of bias point,

FIG. 8 is a block diagram showing an optical transmitter in a firstpreferred embodiment according to the invention,

FIG. 9 is diagrams showing a circuit composition of a precoder 7 in FIG.8 and an input-output logic table therein,

FIGS. 10A to 10D are diagrams showing the relation among the input andoutput of the precoder 7 and the amplitude and phase of signal light 4,

FIG. 11 is a block diagram showing an optical transmitter in a secondpreferred embodiment according to the invention,

FIG. 12 is a block diagram showing an optical transmitter in a thirdpreferred embodiment according to the invention,

FIG. 13 is a block diagram showing an optical transmitter in a fourthpreferred embodiment according to the invention,

FIG. 14 is a diagram showing a waveform change by a waveform equalizer18 in FIG. 13,

FIG. 15 is a block diagram showing an optical transmitter in a fifthpreferred embodiment according to the invention,

FIG. 16 is a block diagram showing an optical transmitter in a sixthpreferred embodiment according to the invention,

FIG. 17 is a block diagram showing an optical transmitter in a seventhpreferred embodiment according to the invention,

FIG. 18 is a block diagram showing an optical transmitter in an eighthpreferred embodiment according to the invention,

FIG. 19 is a diagram showing an input-output logic table of aparallel-type precoder 39 in FIG. 18,

FIG. 20 is a diagram showing a first circuit composition of the precoder39,

FIG. 21 is a diagram showing a second circuit composition of theprecoder 39,

FIG. 22 is a block diagram showing an optical transmitter in a ninthpreferred embodiment according to the invention,

FIG. 23 is a block diagram showing an optical transmitter in a tenthpreferred embodiment according to the invention,

FIGS. 24A and 24B are diagrams showing an optical spectrum and amodulated light waveform in the case that optical intensity modulationin the tenth embodiment is conducted at a conventional bias point,

FIGS. 25A and 25B are diagrams showing an optical spectrum and amodulated light waveform in the case that optical intensity modulationin the tenth embodiment is conducted at a shifted bias point,

FIG. 26 is a block diagram showing an optical transmitter in an eleventhpreferred embodiment according to the invention,

FIG. 27 is a block diagram showing an optical transmitter in a twelfthpreferred embodiment according to the invention,

FIG. 28 is a diagram showing an input-output characteristic of asaturation amplifier in FIG. 27,

FIGS. 29A to 29C are diagrams showing input and output waveforms and anoptical spectrum of the saturation amplifier,

FIG. 30 is a block diagram showing an optical transmitter in athirteenth preferred embodiment according to the invention,

FIG. 31 is a diagram showing an input-output characteristic of a circuitcomposed of a diode and a amplifier in FIG. 30,

FIG. 32 is a block diagram showing an optical transmitter in afourteenth preferred embodiment according to the invention, and

FIG. 33 is a block diagram showing an optical transmitter in a fifteenthpreferred embodiment according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method for generating a duobinary signal and an optical transmitterusing the same method in the first preferred embodiment will beexplained in FIG. 8. The first embodiment is adapted to an opticaltransmitter in an 10 Gb/s optical duobinary modulation-direct detectionreception manner. As shown in FIG. 8, the output of a 1.5 μm bandsemiconductor laser 1 is input to a Mach-Zehnder(MZ) optical intensitymodulator 2 which is composed of an lithium niobate(LiNbO₃, hereinafterreferred to as `LN`) optical waveguide, and then its output is input toa LN optical phase modulator 3. The LN optical intensity modulator 2turns ON or OFF the optical output according to the value, 1 or 0, of anelectrical signal to be input. Also, the LN optical phase modulator 3modulates the optical phase into π or 0 according to the value, 1 or 0,of an electrical signal to be input. A 10 Gb/s data signal 6 is dividedinto two signals, one of which is input to the LN optical intensitymodulator 2 to intensity-modulate signal light, and the other of whichis input to a precoder 7 and is encoded based on the relation that theoutput is inverted when the input is 0, as described earlier. An exampleof a circuit composition of the precoder 7 and an input-output logictable are shown in FIG. 9. FIGS. 10A to 10D illustrate the operation ofthe precoder 7 and the relation between the amplitude and phase of themodulated signal light 4. The encoded data signal 8 is input to a0.5-bit delay circuit 9 and delayed therein, thereafter input to the LNoptical phase modulator 3 to intensity-modulate the signal light.Meanwhile, the 0.5-bit delay circuit 9 is so adjusted that, consideringthe propagation delay time from the LN optical intensity modulator 2 tothe LN optical phase modulator 3, the 0.5-bit delay phase relationbetween the intensity modulation and the phase modulation is obtained.

As the result of the optical modulation by the above composition andprocess, the measured full width at half maximum of the optical spectrumof the signal light 4 output is 5 Ghz. When signal light by standardintensity modulation is output stopping the phase modulation in the samecomposition, the full width at half maximum of the optical spectrum isabout 10 Ghz. Thus, it is proved that the bandwidth is reduced to 1/2due to the invention. When the signal light 4 is transmitted through a1.3 μm zero-dispersion optical fiber with a length of 150 km, thedispersion deterioration after the transmission is less than 1 dB. Thestandard intensity-modulated signal light cannot be received at 50 kmdue to the waveform deterioration. Namely, it is proved that the signallight 4 produced according to the invention is durable against thedispersion.

A method for generating a duobinary signal and an optical transmitterusing the same method in the second preferred embodiment will beexplained in FIG. 11. The second embodiment is given as an example usingan integrated light source 11 where an optical intensity modulator andan optical phase modulator are fabricated using semiconductor and areintegrated with a distributed feedback(DFB) semiconductor laser. Theoptical intensity modulator used is anelectric-field-absorption-type(EA) optical intensity modulator. Theoptical phase modulator used is a modulator of the type that the phasemodulation is conducted by using the effect that a refractive index insemiconductor is varied by the application of electric field.

As the result of the modulation experiment at 10 Gb/s using theintegrated light source 11, it is proved that the spectrum width and thedispersion deterioration characteristic which are similar to those inthe first embodiment are obtained. Also, the integration of the lightsource and the optical modulator enables the miniaturization of theoptical transmitter.

A method for generating a duobinary signal and an optical transmitterusing the same method in the third preferred embodiment will beexplained in FIG. 12. The third embodiment is given as an example usinga DFB/EA integrated light source 15 where an optical intensity modulatorand a DFB semiconductor laser are integrated. Instead of the phasemodulation, used is the direct modulation phase shift keying(PSK)technique where the optical phase modulation is conducted by pulsativelymodulating the injection current of the DFB semiconductor laser andoptical-frequency modulating. A direct modulation PSK encoder 16generates pulses with a width shorter than 1 time slot at the rising andfalling of data series to be phase-modulated, therebyoptical-frequency-modulating the signal light. By adjusting the degreeof frequency modulation, the modulation equivalent to the 0-π phasemodulation is conducted.

As the result of the modulation experiment at 10 Gb/s using theintegrated light source 15, it is proved that the spectrum width and thedispersion deterioration characteristic which are similar to those inthe first and second embodiments are obtained. Also, the integration ofthe light source and the optical modulator enables the miniaturizationof the optical transmitter.

A method for generating a duobinary signal and an optical transmitterusing the same method in the fourth preferred embodiment will beexplained in FIG. 13. The fourth embodiment is given as an example wherea data signal 6 to be input to an optical intensity modulator 2 iswaveform-transformed by a waveform equalizer 18. Examples of an inputwaveform and an output waveform of the waveform equalizer 18 are shownin FIG. 14. As shown in FIG. 14, the cross point of data signal is movedclose to a level of 1 and a level of zero is pointed properly. By thetransformation of waveform, the variation of phase and amplitude at thelevel of zero becomes closer to that of the ideal optical duobinarysignal light, and the durability against the dispersion can be enhanced.

As the result of the transmission at 10 Gb/s using the waveformequalizer 18, it is proved that, when the signal light 4 is transmittedthrough a 1.3 μm zero-dispersion optical fiber with a length of 200 km,the dispersion deterioration after the transmission is less than 1 dB.In contrast with this, when the waveform equalizer 18 is not used, thereoccurs a deterioration of 1 dB for about 150 km transmission.

A method for generating a duobinary signal and an optical transmitterusing the same method in the fifth preferred embodiment will beexplained in FIG. 15. The fifth embodiment is given as an example wherean optical intensity modulator and an optical phase modulator are givenby a push-pull type MZ modulator 19 which is integrated on a LNsubstrate. By modulating each of arms of the push-pull optical modulator19 with V.sub.π, it operates as an optical phase modulator, and, bymodulating each of arms of the push-pull optical modulator 19 withV.sub.π /2, it operates as an optical intensity modulator. Also, it canoperate as an ideal modulator that higher harmonic components aresuppressed since unnecessary chirp does not occur.

As the result of the modulation experiment at 10 Gb/s by the abovecomposition, it is proved that, as compared with the first to fourthembodiments, components near the base of the optical spectrum are bestsuppressed and higher harmonic components are suppressed.

A method for generating a duobinary signal and an optical transmitterusing the same method in the sixth preferred embodiment will beexplained in FIG. 16. The sixth embodiment is given as an example usinga module 28 where the integrated light source 11 composed of the opticalintensity modulator, optical phase modulator and DFB semiconductor laserin the second embodiment in FIG. 11 is airtightly sealed. In theintegrated light source 11a, the positions of the optical intensitymodulator and the optical phase modulator are reverse to those of theintegrated light source 11. Also, a D-type flip-flop 25, where an outputQ and an inverted output Q(⁻, bar) are obtained, is used to divide thedata signal 6. Thereby, the effect of waveform adjustment as well as thedividing can be obtained. Meanwhile, an EX-OR circuit 26 is used sincethe precoder inputs the inverted output Q(bar). Also, to easily adjustthe phase relation of modulated signals, at an intensity modulationterminal 29 and a phase modulation terminal 30 of the integrated lightsource module 28, delay time is adjusted by a microstrip delay line 31so that the difference between the propagation delay and the wiringdelay of light in the integrated light source 11a is zero between theintensity-modulated signal and phase-modulated signal.

As the result of the modulation experiment at 20 Gb/s by the abovecomposition, the operation is well conducted. Also, the high-speedoperation can be performed by shortening the wiring and using the smallmodule.

A method for generating a duobinary signal and an optical transmitterusing the same method in the seventh preferred embodiment will beexplained in FIG. 17. The seventh embodiment is given as an examplewhere the precoder is composed of a counter 35. Q(bar) output of aD-type flip-flop 25 is input to an ENABLE terminal of the binary counter35. Namely, when the data signal 6 is 0, Q(bar) becomes 1 and then thecounter 35 counts a clock signal 24. As a result, output Q₀, whichrepresents the first place of the counter 35, shifts alternately between1 and 0. Thus, it can conduct the same operation as the precoder 7 inFIG. 9.

As the result of the modulation experiment at 10 Gb/s by the abovecomposition, it is proved that it operates like the case using theprecoder composed of EX-NOR.

A method for generating a duobinary signal and an optical transmitterusing the same method in the eighth preferred embodiment will beexplained in FIG. 18. The eighth embodiment is given as an example wheretwo optical intensity modulators are parallel disposed, whereby twodivided lights are switched individually, thereafter coupled to givesignal light 4 with a phase difference of π. First, the output of asemiconductor laser 1 is input to a parallel-type optical modulator 36.Then, the light input is divided into the two lights in theparallel-type optical modulator 36, then input to the first and secondoptical intensity modulators 38a and 38b, respectively. One of theoutputs of the first and second optical intensity modulators 38a, 38b isphase-shifted by a π optical phase shifter 37 to give the phasedifference of π between the two output lights, thereafter coupled. Theentire parallel-type optical modulator 36 is formed as a MZinterferometer. The data signal 6 is passed through the precoder 7 likethat shown in FIG. 9, then input to a parallel-type precoder 39, thereinconverted into first and second encoded data signals 40a, 40b, which areinput to the first and second optical intensity modulators 38a and 38b,respectively. The input-output relations in the parallel-type precoder39 are shown in FIG. 19. The first and second encoded data signals 40a,40b are, as shown in FIG. 18, represented by Q₀ and Q.sub.π,respectively. From the current data signal D(i) and data signal 1-bitbefore D(i-1), the analog addition, (D(i)+D(i-1)), gives three values of0, 1 and 2. The three values are mapped to three states, (1, π), (0,unfixed) and (1,0), where the amplitude and phase of light arerepresented by (A, Φ). This is conducted by using the combination of Q₀and Q.sub.π in FIG. 19. Meanwhile, when both Q₀ and Q.sub.π are 1 andlights on both the arms are ON, the phase difference between the lightsis π. Therefore, the power of the output light becomes zero wheninterfered at the coupling position. The input and output of theparallel-type precoder 39 as shown in FIG. 19 are given by:

    Q.sub.0 =D(i),Q.sub.π =D(i-1)

Circuit examples of the parallel-type precoder 39 are shown in FIGS. 20and 21.

As the result of the modulation experiment at 10 Gb/s by the abovecomposition, it is proved that a spectrum width and a dispersiondeterioration characteristic equivalent to those in the signal lightobtained by the cascade modulation in the first to seventh embodimentsare obtained.

A method for generating a duobinary signal and an optical transmitterusing the same method in the ninth preferred embodiment will beexplained in FIG. 22. The ninth embodiment is given as an example usinga parallel-type push-pull optical modulator 41 where push-pull MZinterferometers are disposed on both the arms of a MZ interferometer. Abias electrode 43 is disposed on the optical waveguide of one of thearms, where the optical phase is shifted by π by applying a voltage.Though, in FIG. 22, a bias-voltage applying circuit for the push-pull MZinterferometers disposed on both the arms is not shown, the bias voltageis so adjusted that the intensity modulation is optimumly conducted byboth the MZ interferometers. The outputs Q, Q(bar) of a D-type flip-flop25 are individually divided into two signals. The push-pull MZ modulatorfor phase-zero signal light is driven Q and Q(bar), and the push-pull MZmodulator for phase-π signal light is driven by 1-bit delayed Q andQ(bar). As a result, it can operate like the logic of the parallel-typeprecoder 39 as shown in FIG. 19.

As the result of the modulation experiment at 10 Gb/s by the abovecomposition, it is proved that it operates stably like the eighthembodiment.

Though the invention is explained by above embodiments, the invention isnot limited to their compositions and can receive various modifications.

Also, the invention can be applied to any wavelength, while the aboveembodiments employ the 1.5 μm wavelength-band semiconductor laser.Furthermore, any laser, such as a gas laser, a solid-state laser and anorganic laser, other than the semiconductor laser is applicable. Thecarrier wave can be any electromagnetic wave, such as microwave andmillimetric wave, other than light.

Though the above experiments are conducted at the bit rates of 10 and 20Gb/s, the bit rate may be higher or lower than these.

Also, any material of the optical intensity modulator can be used, whilethe optical intensity modulators in the embodiments use LN andsemiconductor. Furthermore, other than the MZ-type and electric fieldabsorption(EA) type modulators, any optical intensity modulators, suchas acousto-optic effect type, electro-optic effect type,polarization-rotation type and non-linear effect type, which canmodulate an optical intensity according to an input signal, may be used.Also, the input signal is not limited to an electrical signal, e.g., anoptical intensity modulator controlled by light may be used. Also in theoptical phase modulator, the material, composition, effect to beemployed etc. are not limited, i.e., any type of optical phasemodulators, which can modulate an optical phase according to an inputsignal, may be used.

Though the circuit examples of the precoder are shown in the aboveembodiments, any logic circuit, such as AND, OR and flip-flop, includingan analog circuit, may be used.

Also, the phase inversion may be omitted when an amplitude of zero occurcontinuously, while the phase is always inverted when the amplitude oflight is zero in the embodiments.

Though the 0.5-bit delay circuit 9 serves to delay by 0.5 bit the timingof intensity modulation and phase modulation, this delay value is notlimited. Namely, when there exists a propagation delay in the connectioncable, the delay value needs to be adjusted shifting from 0.5 bit. Theposition of the 0.5-bit delay circuit 9, which is located after theprecoder in the embodiments, may be located before the precoder.Alternatively, an optical delayer may be used to delay by 0.5 bit in theoptical region. The delay may be adjusted wherever the timing ofintensity modulation and phase modulation can be shifted by 0.5 bit. The1-bit delay circuit 27 can be modified as well.

Though the π optical phase shifter 37 serves to shift by π the phasedifference between the two divided signal lights, this shift value isnot severely limited when there exists an optical-path-length differenceor birefringence on both the arms of the MZ interferometer. The π phaseshift may be achieved by using the optical-path-length difference orbirefringence in the optical waveguide, refractive-index variation ofthe modulator material by applying a current or electric field,non-linear effect etc. Also, the position of the π optical phase shifter37 may be before the optical intensity modulator 38a or 38b, and theoptical dividing or coupling part may have this function. Also, a phasedifference of -π may be used.

A method for generating a duobinary signal and an optical transmitterusing the same method in the tenth preferred embodiment will beexplained in FIG. 23. The tenth embodiment is given as an example where,in the duobinary optical transmitter of the first embodiment in FIG. 8,the bias of the MZ modulator is adjusted to obtain an ideal duobinarywaveform. An optical intensity modulator 2 is provided with a biascircuit 66 which sets a modulation operating point. The bias is, asshown in FIG. 6B, set to be shifted to the side of the transmission peakfrom the center of the modulation characteristic 61 of the modulator.The amount of shifting is preferably about 10 to 20% of V.sub.π.Thereby, the cross point of the optical waveform after the intensitymodulation is shifted to the side of optical transmission to be close tothe ideal duobinary signal waveform. The measurement results forspectrum and optical waveform of 10 Gbps duobinary modulation lightgenerated by the above composition are shown in FIGS. 24A and 24B,respectively. On the other hand, the measurement results for spectrumand optical waveform in the case that the average of a voltage signal tobe applied to the LN optical intensity modulator is conventionally setto be the center of the extinction curve are shown in FIGS. 25A and 25B,respectively. Comparing FIG. 24B with FIG. 25B, it is proved that, inthis embodiment, the high-frequency components higher than 5 Ghz issuppressed.

A method for generating a duobinary signal and an optical transmitterusing the same method in the eleventh preferred embodiment will beexplained in FIG. 26. The eleventh embodiment is given as an examplewhere, to the duobinary optical modulator in FIG. 23, an opticalspectrum analyzer 67 for detecting the spectrum width of light outputfrom the modulator, a computer 68 for processing the data from theoptical spectrum analyzer 67 to calculate the spectrum width ofmodulated light, and a control circuit 69 for controlling a voltageapplied to a bias circuit 66 to minimize the spectrum width. In thiscomposition, the bias to the optical intensity modulator 2 is controlledto minimize the spectrum width measured by the optical spectrum analyzer67. Therefore, even when the modulation characteristic of the opticalintensity modulator 2 is varied, the spectrum width is always kept to beminimum and the stable operation is thereby obtained.

A method for generating a duobinary signal and an optical transmitterusing the same method in the twelfth preferred embodiment will beexplained in FIG. 27. The eleventh embodiment is given as an examplewhere the modulation optical waveform is brought close to an idealduobinary waveform by using a non-linear input-output characteristic ofa saturation amplifier 70. The saturation amplifier 70 has theinput-output characteristic 71 as shown in FIG. 28. When a binary signalas shown in FIG. 29A is input to this circuit, the cross point of theoutput signal is biased upward as shown in FIG. 29B, whereby the outputsignal close to the ideal duobinary modulation waveform is obtained.When the optical duobinary signal is generated by driving the opticalintensity modulator 2 through this electrical signal, the operatingpoint is so set that light is transmitted through when the input voltageis high and light is intercepted when the input voltage is low. By thissetting, the cross point of the optical waveform after modulation isbiased to the side of optical transmission and the high-frequencycomponents of optical spectrum is reduced as shown in FIG. 29C.

Meanwhile, an electric circuit having a non-linear input-outputcharacteristic is not limited to the saturation amplifier. For example,a combination of a diode and an amplifier can be used.

A method for generating a duobinary signal and an optical transmitterusing the same method in the thirteenth preferred embodiment, whichemploys such a combination, will be explained in FIG. 30. Theinput-output characteristic 74 of a circuit where a diode 72 and anamplifier 73 are connected in series is shown in FIG. 31. Contrary tothe case using the saturation amplifier, the gain is lowered where theinput voltage is low. Therefore, the cross point of the output waveformis biased downward. Using this waveform, the optical intensity modulator2 is so operated that light is intercepted when the input voltage ishigh and light is transmitted through when the input voltage is low. Bythis setting, the cross point of the optical waveform after modulationis biased to the side of optical transmission and the high-frequencycomponents of optical spectrum is reduced.

Though, in the twelfth and thirteenth embodiments, the operation stateof the optical intensity modulator is defined according to themodulation characteristic curve, it will be easily appreciated that eventhe reverse operation state of the optical intensity modulator isapplicable by inverting the modulated waveform by inserting an invertingamplifier after the non-linear circuit.

A method for generating a duobinary signal and an optical transmitterusing the same method in the fourteenth preferred embodiment will beexplained in FIG. 32. The fourteenth embodiment is given as an examplewhere means for detecting an optical spectrum width is provided for theoutput of the optical intensity modulator in the twelfth embodiment tocontrol the bias of an input signal to the non-linear electric circuitto minimize the spectrum width. The spectrum width detecting means iscomposed of an optical spectrum analyzer 67, a data processing computer68 and a control circuit 69, like the eleventh embodiment. The controlcircuit 69 controls the bias applied to the input signal to thenon-linear circuit to minimize the spectrum width to be calculated bythe computer 68. Thereby, even when the input-output characteristic ofthe electric circuit is varied, the modulated waveform is always kept tohave a narrow spectrum width.

A method for generating a duobinary signal and an optical transmitterusing the same method in the fifteenth preferred embodiment will beexplained in FIG. 33. The fifteenth embodiment is given as an examplewhere the components of the first embodiment are used as they are andthe LN optical intensity modulator 2 and the LN optical phase modulator3 are so connected that their optical axes are inclined by 45° to eachother. By inputting light while inclining by 45° the main axis of thepolarization of signal light 4 to the optical axis of the optical phasemodulator 3, only the component in the direction of the optical axis ofthe signal light 4 is phase-modulated and the component in the directionorthogonal to the optical axis of the signal light 4 is notphase-modulated. As a result, the polarization state of the signal lightis polarization-modulated according to a driving signal to the opticalphase modulator 3. The signal light is the sum of the phase-modulatedduobinary signal and the non-phase-modulated intensity modulation signallight. Though it is not a perfect duobinary signal, the optical spectrumis a little narrowed since the component of the intensity modulationlight is halved. Furthermore, since the signal light ispolarization-modulated, a variation in amplification factor(polarizationhole burning) depending upon polarization to be supplied etc., which isa problem of a system using an optical amplifier, can be suppressed.

As the result of the optical modulation by the above composition andprocess, the measured full width at half maximum of the optical spectrumof the signal light 4 output is 7.5 GHz. When signal light by standardintensity modulation is output stopping the phase modulation in the samecomposition, the full width at half maximum of the optical spectrum isabout 10 Ghz. Thus, it is proved that the bandwidth is reduced due tothe invention. When the signal light 4 is input and transmitted throughan optical amplification repeating system with a total length of 1000 kmwhich is composed of optical amplification repeaters disposed atintervals of 50 km optical fiber, it is proved that the polarizationhole burning of the optical amplification repeater is suppressed due tothe polarization modulation and that the optical signal-to-noiseratio(optical SNR) is improved by 3 dB compared with a case without thepolarization modulation.

Though, in the fifteenth embodiment, the optical phase modulator 3 whoseoptical axis direction is inclined by 45° is used, an opticalpolarization modulator is not limited to this. For example, it may becomposed by dividing the signal light into two polarized waves that areorthogonal to each other, then phase-modulating only one of the signallight through the optical phase modulator, again coupling them. Also, itis not limited to the LN phase modulator. The material may be ofsemiconductor, organic, inorganic, an optical fiber etc. if it is usablefor the high-speed polarization modulation. The modulation manner may beelectrical, magnetical, mechanical, optical etc.

Although the invention has been described with respect to specificembodiment for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodification and alternative constructions that may be occurred to oneskilled in the art which fairly fall within the basic teaching here isset forth.

What is claimed is:
 1. A method for generating a duobinary signal,comprising the steps of:dividing a binary data signal into a firstbinary data signal and a second binary data signal; inputting said firstbinary data signal into an optical intensity modulator; inputting saidsecond binary data signal into an optical phase modulator; andmodulating individually an intensity of a carrier wave, using saidoptical intensity modulator, and a phase of said carrier wave, usingsaid optical phase modulator, and outputting a duobinary signal fortransmission on said carrier wave.
 2. A method for generating aduobinary signal, according to claim 1, wherein:said phase is modulatedby shifting by π when said intensity is zero.
 3. A duobinary-manneroptical transmitter, comprising:a laser device which outputs signallight; an optical intensity modulator which intensity-modulates saidsignal light according to a first binary data signal; a precoder whichinputs a second binary data signal; and an optical phase modulator whichphase-modulates said intensity-modulated signal light according to abinarm signal which is obtained by delaying by 0.5 bit an output binarysignal of said precoder.
 4. A duobinary-manner optical transmitter,according to claim 3, wherein:said precoder gives its output invertedfrom 0 to 1 or from 1 to 0 when said data signal to be input is 0 andholds its output as it is when said data signal to be input is
 1. 5. Aduobinary-manner optical transmitter, according to claim 3, wherein:atleast two of said laser device, said optical intensity modulator andsaid optical phase modulator are integrated.
 6. A duobinary-manneroptical transmitter, according to claim 3, wherein:said first datasignal is input to said optical intensity modulator after its waveformis varied by a waveform equalizer.
 7. A duobinary-manner opticaltransmitter, according to claim 3, wherein:at least one of said opticalintensity modulator and said optical phase modulator is a push-pull typeoptical modulator.
 8. A duobinary-manner optical transmitter, accordingto claim 3, wherein:said optical intensity modulator and said opticalphase modulator are reversely cascade-connected.
 9. A duobinary-manneroptical transmitter, comprising:a laser device which outputs signallight; an optical intensity modulator which intensity-modulates saidsignal light according to a first binary data signal; a precoder whichinputs a second binary data signal; and an optical phase modulator whichphase-modulates said intensity-modulated signal light according to abinary signal which is obtained by delaying by 0.5 bit an output binarysignal of said precoder; wherein a waveform of said signal light isvaried by changing an operating point of said optical intensitymodulator.
 10. A duobinary-manner optical transmitter, according toclaim 9, further comprising:means for measuring an optical spectrum ofoutput light; means for calculating a spectrum width from measurementdata output from said optical spectrum measuring means; and means forcontrolling said operating point of said optical intensity modulator;wherein said operating point of said optical intensity modulator iscontrolled by said controlling means to minimize said spectrum width.11. A duobinary-manner optical transmitter, according to claim 9,further comprising:a device for measuring an optical spectrum of outputlight; a calculator for calculating a spectrum width from measurementdata output from said optical spectrum measuring device; and acontroller for controlling said operating point of said opticalintensity modulator; wherein said operating point of said opticalintensity modulator is controlled by said controller to minimize saidspectrum width.
 12. A duobinary-manner optical transmitter, comprising:alaser device which outputs signal light; an optical intensity modulatorwhich intensity-modulates said signal light according to a first binarydata signal; a precoder which inputs a second binary data signal; and anoptical phase modulator which phase-modulates said intensity-modulatedsignal light according to a binary signal which is obtained by delayingby 0.5 bit an output binary signal of said precoder; wherein a waveformof said first binary data signal is varied through a non-linear electriccircuit.
 13. A duobinary-manner optical transmitter, according to claim12, further comprising:means for measuring an optical spectrum of outputlight; means for calculating a spectrum width from measurement dataoutput from said optical spectrum measuring means; and means forcontrolling a bias of said first data signal to be input to saidnon-linear electric circuit; wherein said bias is controlled by saidbias controlling means to minimize said spectrum width.
 14. Aduobinary-manner optical transmitter, according to claim 12, furthercomprising:a device for measuring an optical spectrum of output light; acalculator for calculating a spectrum width from measurement data outputfrom said optical spectrum measuring means; and a bias controller forcontrolling a bias of said first data signal to be input to saidnon-linear electric circuit; wherein said bias controller minimizes saidspectrum width.
 15. A duobinary-manner optical transmitter, comprising:alaser device which outputs signal light; an optical intensity modulatorwhich intensity-modulates said signal light according to a first binarydata signal; a precoder which inputs a second binary data signal; and aphase modulator which phase-modulates said signal light according tosaid second binary data signal.